martin manning wrote:Nice re-purposing going on here. I'm confused by the markings on the OT saying that the primaries are 22k? That doesn't make sense. The donor amp had what kind of tubes? The load should be ~4k for a pair of 6L6.
Thanks Martin.
The markings are incorrect. The iron came from a Blues Custom 30. 2 6L6 and 12AX7 pres.
Here are my numbers...if I did the math correctly it's 7.2K. I used a 9vac wall wart on the 8Ω tap. I measured 278vac on the primaries.
xtian wrote:How did you get a nice straight cut on your panel cutout? Did you just use a jigsaw?
I free handed it on the band saw to get clearance and then set up a fence. I set the long edge against the fence at a 45° angle, started the saw and then laid the piece down into the blade. Pushed along the fence to take off the last 1/16th. Then cleaned it up on an oscillating belt sander.
Guitarnut wrote:Here are my numbers...if I did the math correctly it's 7.2K. I used a 9vac wall wart on the 8Ω tap. I measured 278vac on the primaries.
Comes out to 30:1
30*30=900
900*8=7200 or 7.2K
7.2k is an odd impedance for 6L6's. Did you measure the wall-wart voltage at the same time as you measured the primary voltage, with the wall-wart hooked up to the OT secondary? And, do I assume correctly that you measured the primary voltage across the two plate leads, not CT to plate?
Guitarnut wrote:Here are my numbers...if I did the math correctly it's 7.2K. I used a 9vac wall wart on the 8Ω tap. I measured 278vac on the primaries.
Comes out to 30:1
30*30=900
900*8=7200 or 7.2K
7.2k is an odd impedance for 6L6's.
GN, this was why I was asking if the OT was native 7k2 and if you were gonna switch secondary taps to halve the primary impedance to be closer to the usual 4k primary for 6L6s. 7k6 to 8k is more typical of 6V6 and EL84s. Nice work on the cab!
martin manning wrote:
7.2k is an odd impedance for 6L6's. Did you measure the wall-wart voltage at the same time as you measured the primary voltage, with the wall-wart hooked up to the OT secondary? And, do I assume correctly that you measured the primary voltage across the two plate leads, not CT to plate?
I didn't meter the wall wart while it was attached. I went by it's output rating. Guess I should have checked it. I metered both plate leads.
Easy enough to set up and check it again.
Peace,
Mark
Last edited by Guitarnut on Mon Jun 10, 2013 11:10 pm, edited 1 time in total.
Colossal wrote:
GN, this was why I was asking if the OT was native 7k2 and if you were gonna switch secondary taps to halve the primary impedance to be closer to the usual 4k primary for 6L6s. 7k6 to 8k is more typical of 6V6 and EL84s. Nice work on the cab!
The Epi Blues Custom had a pentode/triode switch. I believe in triode mode two of the screens were sent to ground. Would this be why the odd impedance OT was used?
Peace,
Mark
EDIT: Just reviewed the schematic for the BC30...I'm not sure what's going on in triode mode.
Last edited by Guitarnut on Mon Jun 10, 2013 11:37 pm, edited 1 time in total.
If the primary is 7k2 with an 8-ohm load on the 8-ohm secondary tap, you could connect the 8-ohm load to the 16-ohm secondary tap, and get a 3k6 reflected load on the primary.
Re the triode connection, do you have a schematic of the donor amp?
martin manning wrote:If the primary is 7k2 with an 8-ohm load on the 8-ohm secondary tap, you could connect the 8-ohm load to the 16-ohm secondary tap, and get a 3k6 reflected load on the primary.
Re the triode connection, do you have a schematic of the donor amp?
Thanks. That's what I was thinking when Colossal asked earlier.
What I've read about OT impedance is that with a higher impedance reflected back to the primary, the tubes would produce less power. Maybe this is why the BC30 was rated at 30W/15W off two 6L6s.
I've always wondered about the 250R/25W on the BC30 power tube cathodes. Is this some sort of power soak?
Last edited by Guitarnut on Mon Jun 10, 2013 11:33 pm, edited 1 time in total.
Guitarnut wrote:Thanks! I was beginning to think I was just talking to myself.
LOL I was beginning to think the same thing. Though I've been enjoying this thread from the beginning. I love your cab, you've built a nice set of woodworking skills for yourself!
Guitarnut wrote:Thanks! I was beginning to think I was just talking to myself.
LOL I was beginning to think the same thing. Though I've been enjoying this thread from the beginning. I love your cab, you've built a nice set of woodworking skills for yourself!
Thanks! After building guitars for years, cabs are a walk in the park.
It's cathode biased, which will reduce the output. The pentode-triode switch is the usual type where both screens are connected to the anode (not to ground!). Measure that OT again.
Edit: the 250R is the mechanism for cathode biasing, and it reduces the available plate-cathode voltage and therefore power output.
Last edited by martin manning on Mon Jun 10, 2013 11:40 pm, edited 1 time in total.
